THE EFFECT OF EXPOSURE TO ENHANCED UV-B RADIATION ON THE PENETRATION OF MONOCHROMATIC AND POLYCHROMATIC UV-B RADIATION IN LEAVES OF BRASSICA-NAPUS

Using quartz optical fibres, penetration of both monochromatic (310 nm) and polychromatic UV-B (280-320 nm) radiation in leaves of Brassica napus L. (cv. Ceres) was measured. Plants were grown under either visible light (750 mumol m-2 s-1 photosynthetically active radiation) or with the addition of 8.9 kJ M 2 day 1 biologically effective UV-B (UV-B(BE)) radiation. Results showed that of the 310 nm radiation that penetreated the leaf, 90% was within the initial one third of the leaf with high attenuation in the leaf epidermis, especially in UV-treated plants. Polychromatic UV-B radiation, relative to incident radiation, showed a relatively uniform spectral distribution within the leaf except for collimated radiation. Over 30% of the UV-screening pigments in the leaf, including flavonoids, were found in the adaxial epidermal layer, making this layer less transparent to UV-B radiation than the abaxial epidermis, which contained less than 12% of the UV-screening pigments. UV-screening pigments increased by 20% in UV-treated leaves relative to control leaves. Densely arranged epicuticular wax on the adaxial leaf surface of UV-treated plants may have further decreased penetration of UV-B radiation by reflectance. An increased leaf thickness, and decreases in leaf area and leaf dry weight were also found for UV-treated plants.